Governments and manufacturers are cooperating to improve traffic and vehicle safety using vehicular ad-hoc networks (VANETs), e.g., as specified by the IEEE 802.11p and IEEE P1609 standards. Other standards, such as continuous air-interface, long and medium range (CALM) can also be used. Vehicles in VANETS broadcast traffic and vehicle information, such as a location, velocity, acceleration, and braking status in periodic heartbeat messages, typically every 100 milliseconds.
The Federal Communications Commission (FCC) has allocated a 75 MHz bandwidth at 5.9 GHz for intelligent traffic system (ITS) applications such as VANETS. The bandwidth is allocated exclusively for vehicle-to-vehicle (V2V) communications and vehicle-to-infrastructure (V2I) communications. Dedicated short range (≈0.3 to 1 km) communications (DSRC) has been adopted as a technique for ITS services on this bandwidth.
The bandwidth is partitioned into multiple channels, e.g., seven 10 MHz channels including a control channel (CCH) and six service channels (SCH). The CCH CH178 is only used for public safety and control purposes. No private services are allowed on the CCH. The six SCH service channels are CH172, CH174, CH176, CH180, CH182, and CH184. Channels CH174, CH176, CH180, and CH182 are used for public safety and private services. Channels CH172 and CH184 are allocated as dedicated public safety channels, V2V public safety channel and intersection public safety channel, respectively. It should be noted that other channel partitioning schemes can be used.
Transmit powers limits are defined for the channels. CH178 has two transmission power limits, 33 dBm for non-emergency vehicles, and 44.8 dBm for emergency vehicles. For the middle range service channel CH174 and CH176, the transmission power limit is 33 dBm. For the short range service channel CH180 and CH182, the transmission power limit is 23 dBm. For dedicated public safety channels CH172 and CH184, the transmission power limits are 33 dBm and 40 dBm, respectively.
DSRC is standardized in a Wireless Access in Vehicular Environments (WAVE) protocol according to the IEEE 802.11p and IEEE P1609 standards. For channel coordination and channel synchronization, WAVE partitions time into 100 millisecond Sync Intervals. Each Sync Interval is further partitioned into a 50 milliseconds control channel interval (CCHI), and a 50 milliseconds service channel interval (SCHI). A 4 millisecond Guard Interval (GI) at the beginning of each channel interval accommodates variations in timing.
During the CCHI, high priority messages are broadcasted on the CCH while all transceivers monitor the CCH. The messages can be broadcasted on any channel during the SCHI. In a multi-channel wireless communication network, it is more difficult to reliably broadcast high priority messages than in a single channel network where all transceivers use a common channel all of the time.
WAVE imposes a 54 millisecond latency due to the existence of SCHI and Guard Interval. If an event is detected near the beginning of the SCHI, it takes at least 54 milliseconds to receive the corresponding message during the next CCHI. Even if the message is broadcasted immediately on current operation channel, the latency can still be at least 54 milliseconds for transceivers using different channels. A vehicle moving at 100 km/h travels 1.5 meters in 54 milliseconds, which is long enough to cause an accident. Therefore, a latency of 54 milliseconds is unacceptable.
The FCC has established three priority levels for ITS messages: safety of life, public safety, and non-priority. The lower priority messages can tolerate transmission latency, while high priority messages cannot. Based on the three priority levels, the SAE J2735 standard defines formats for a la carte message, a basic safety message, a common safety request message, an emergency vehicle alert message, and a generic transfer message.
The basic safety message contains safety-related information that is periodically broadcast. The common safety request message allows for specific vehicle safety-related information requests to be made that are required by vehicle safety applications. The emergency vehicle alert message is used for broadcasting warnings that an emergency vehicle is operating in the vicinity. The probe vehicle data message contains status information about the vehicle for different periods of time that is broadcasted to roadside equipment. The a la carte and generic transfer messages allow for flexible structural or bulk message exchange.
Of particular concern to the invention are high priority messages, such as crash-pending notification, hard brake, and control loss, which can only have a latency of up to 10 milliseconds. Other warning messages can have a latency up to 20 milliseconds, e.g., emergency vehicle approaching The messages, such as probe and general traffic information, can have a latency of more than 20 milliseconds.
The 54 milliseconds or greater latencies in the WAVE standard do not satisfy latency requirements of the SAE. Therefore, the latency in WAVE networks needs to be reduced.